The present invention relates to a spectacle edging assist device and a spectacle lens suitability determining method which are used to select spectacle lenses suitable for a spectacle frame.
In general, spectacles are formed such that the pupils of a spectacle wearer and the optical centers of lenses coincide with each other when viewed from the front side. To attach lenses onto the spectacle frame of spectacles of this kind, target positions (eyepoints) corresponding to the pupil positions are set on the spectacle frame first. These target positions are set using the pupillary distance of the spectacle wearer. That is, positions spaced apart from each other by the pupillary distance to have, as their center, the horizontal center of the spectacle frame are set as the target positions.
Next, a lens having a size that can be attached onto the rim portion of the spectacle frame is selected. This lens is a disk-shaped uncut lens. The lens selected by this lens selection operation is a lens having a minimum outer diameter larger than the outer diameter of the rim portion while its optical center is positioned at the target position. This lens selection operation is performed using a spectacle edging assist device as described in, for example, Japanese Patent Laid-Open No. 8-136869 (Reference 1). The selected lens undergoes so-called framing. In this framing, a disk-shaped uncut lens is transformed into a shape conforming to that of the rim portion of the spectacle frame. This framing is done while the optical center of the lens coincides with the target position.
The spectacle edging assist device disclosed in the Reference 1 includes a base on which a scale is graduated, and a holding member which holds the front portion of the spectacle frame so that it overlaps the scale. A dummy lens is attached on this front portion. That is, this spectacle edging assist device is formed such that the operator can observe the scale through the dummy lens. The scale is graduated on the flat surface the base, and the holding member holds the front portion of the spectacle frame so that it is superposed on this flat surface.
To perform the above-mentioned lens selection operation using this spectacle edging assist device, a layout chart on which indices used to determine the suitability of lenses are drawn is employed. The indices include the shapes of a plurality of lenses having different outer diameters, and the optical centers of these lenses. In the selection operation, the layout chart is mounted on the base first, and the front portion of the spectacle frame is mounted on this layout chart next. Note that the above-mentioned operation may be facilitated by placing, in advance, a mark at the target position on the dummy lens of the spectacle frame. Note also that the layout chart is sometimes called a lens chart or a lens card.
After the operator superposes the spectacle frame on the layout chart, he or she compares the lenses drawn on the layout chart and the rim portion of the spectacle frame with each other to select a lens while moving the layout chart. At this time, the operator selects a lens having a minimum outer diameter larger than the outer diameter of the rim portion of the spectacle frame. The operator then superposes the optical center of the selected lens on the mark to visually confirm that the outer shape of this lens falls outside the rim portion.
In recent years, eyeglasses called, for example, sport glasses are available. The spectacle frame of spectacles of this kind is formed to have an angle of elevation larger than normal spectacles, and is therefore considerably curved to cover the spectacle wearer's face.
The spectacle edging assist device as disclosed in the Reference 1 cannot use a spectacle frame having a frame curve which is as large as those of spectacle frames employed in the above-mentioned sport glasses. This is mainly for the following two reasons. First, it is difficult to accurately mount the layout chart because the spectacle frame is considerably curved.
Note that the frame curve is the average of curves (the curvatures of arcs or spheres) which pass through the right end piece, bridge, and left end piece of a frame. That is, the frame curve means the average curvature of curved surface shapes formed by the frame in portions in which the lenses are framed. On the other hand, the frame curve is likely to interlock with the angle of curvature, so it can also be substituted with the angle of curvature under a predetermined condition.
Second, the rim portion of the spectacle frame is greatly inclined with respect to the layout chart on the base. The layout chart is almost horizontally mounted on the base. On the other hand, the rim portion (dummy lens) of the spectacle lens is inclined to be gradually separated from the layout chart in the direction from the horizontal center of the spectacle frame toward the ear side. The rim portion appears smaller in the horizontal direction than its actual horizontal dimension when viewed from above. Hence, when a lens drawn on the layout chart is selected by observing the layout chart through the rim portion, a lens suitable for a rim portion which appears smaller than its actual size, that is, a lens having an outer diameter smaller than the actual outer diameter of the rim portion is selected. If such a lens is attached onto the rim portion after framing, a space (missing portion) is generated between the rim portion and the lens.